Refining of stainless steel

ABSTRACT

The invention consists of refining a chrome alloy steel melt, comprising the of transferring an initial chrome alloy steel melt obtained from a furnace, and containing approximately 1 to 6 percent carbon, into a crucible at or near 1,300*C, injecting gaseous oxygen in the form of a high velocity stream toward the surface of the melt, while maintaining a pressure of less than one atmosphere over the melt at all times; said gas being injected at a rate controlled to maintain the melt at a maximum temperature of 1,800*C, until the desired low carbon content is approached, then further reducing the oxygen flow rate while slowly mixing said oxygen with an inert gas in increasing concentration until the injected gas contains only inert gas maintaining the melt temperature between approximately 1,600*C and 1,800*C, as the furnace pressure is decreased by pumping, thus reducing the percentage of residual carbon to approximately between 0.002 and 0.08 percent depending upon the percentage of chrome which is between approximately 8 and 20 percent.

United States Patent [191 Umowski Nov. 26, 1974 1 1 REFINING OF STAINLESS STEEL [76] Inventor: Joseph S. Umowski, 83 Sherman Related US. Application Data [63] Continuation-in-part of Ser. No. 64,669, April 14, 1970, abandoned, which is a continuation of Ser. No. 622,961, March 14, 1967, abandoned.

[52] US. Cl 75/49, 75/60, 75/l30.5

[51] Int. Cl. C2lc 7/10 [58] Field of Search 75/49, 59, 60, 130.5

[56] References Cited UNITED STATES PATENTS 2,093,666 9/1937 Vogt 75/49 X 2,624,671 1/1953 Binder 75/49 3,003,865 10/1961 Bridges 75/l30.5 3,046,107 7/1962 Nelson et a1... 75/59 3,252,790 5/1966 Krivsky 75/60 3,307,937 3/1967 Philblad 75/59 3,420,657 1/1969 Hansen 75/49 X 3,666,439 5/1972 Ramachandran 75/49 FOREIGN PATENTS OR APPLICATIONS 15,209 8/1963 Japan 75/49 1,343,235 11/1963 France 75/60 OTHER PUBLICATIONS The Book of Stainless Steels, 2d Edition, Thum,

Amer. Soc. for Metals, Cleveland, 1935, page 61.

Primary ExaminerL. Dewayne Rutledge Assistant Examiner-Peter D. Rosenberg Attorney, Agent, or FirmBlum, Moscovitz, Friedman & Kaplan [57] ABSTRACT The invention consists of refining a chrome alloy steel melt, comprising the of transferring an initial chrome alloy steel melt obtained from a furnace, and containing approximately 1 to 6 percent carbon, into a crucible at or near l,300C, injecting gaseous oxygen in the form of a high velocity stream toward the surface of the melt, while maintaining a pressure of less than one atmosphere over the melt at all times; said gas being injected at a rate controlled to maintain the melt at a maximum temperature of 1,800C, until the desired low carbon content is approached, then further reducing the oxygen flow rate while slowly mixing said oxygen with an inert gas in increasing concentration until the injected gas contains only inert gas maintaining the melt temperature between approximately l,600C and 1,800C, as the furnace pressure is decreased by pumping, thus reducing the percentage of residual carbon to approximately between 0.002 and 0.08 percent depending upon the percentage of chrome which is between approximately 8 and 20 percent.

4 Claims, 5 Drawing Figures REFINING F STAINLESS STEEL This application is a continuation-in-part of Ser. No. 64,669, filed on Apr. 14, 1-970, and now abandoned, which application is a continuation of Ser. No. 622,961, filed on Mar. 14, 1967, and now abandoned.

This invention relates to a new and unique technique for the production of stainless steel by oxygen blowing at sub-atmospheric pressure and in the presence of an inert gas.

One of the primary objects of the present invention is to provide a method of refining molten stainless steel whereby excessive carbon content of the melt is reduced to minimal levels with little or no loss of the chromium content, by controlling the temperature, pressure, carbon and chromium relationship.

A further object of the present invention is to utilize the lowest cost raw materials in the production of a utility type stainless steel for as highway, marine, architectural use, etc., requiring a high chromium content, a low carbon content and an extremely low residual Oxygen content.

Although the invention relates principally to the production of chromium iron alloys such as the 430 type stainless steel, a still further aim of the present invention is to provide the basic technique or process for the production of any other type by the addition of any other desirable elements to produce a specific alloy for a specific use.

A more specific object of the invention is to produce stainless steel at a cost far below the cost of present processes for such items as tools, highway guard rails, auto parts, gutters, fences, architectural trim, etc.

Another specific object of the invention is to permit the use of refining temperatures below those which normally result in a rapid deterioration of the furnace linings.

A further object of the invention is to obtain the predominent heat necessary for the refinement of the melt by the exothermic oxidation of elements such as carbon, silicon, iron, etc. The temperature can be controlled by inductance heating, or cooling with scrap.

Furthermore in accordance with the invention, the oxygen flow is controlled so that practically all of the oxygen is utilized to remove the excess carbon.

Another advantage of the invention is that it involves the use of vacuum furnaces, vacuum induction furnaces and the more recent sealed Basic oxygen furnaces which are well known per se and which facilitate the control of vessel pressure.

Further in accordance with the invention, the refining process may require little or no flux with the result that little or no slag is formed.

In a specific embodiment of the invention the vessel pressure ranges from 0.03 to 0.33 atmospheres and temperatures between 1,600C and l,800C, the subatmospheric pressure used in this process removes dissolved carbon to very low concentration without oxidizing the chromium content to any appreciable amount, permitting production of stainless steel at very low cost with carbon contents as low as 0.002- percent deoxidation.

Theseand other objects of the invention will be more fully apparent from the drawings annexed herein in which FIG. 1 illustrates in principle and schematically an arrangement for refining stainless steel embodying the principles of the invention.

FIG. 2 is a graph illustrating the reduction of carbon content as a function of the application of subatmospheric pressure and within a given relatively low temperature range, on a melt originally containing approximately 16 r percent chromium.

FIG. 3 is a graph illustrating the reduction of carbon content as a function of decreasing absolute pressure within high chrome content.

. FIG. 4 is a graph illustrating the reduction of carbon content as a function of reducing sub-atmospheric pressure at a given, relatively low, chrome content within a narrow, relatively low, temperature range.

FIG. 5 is a graph illustrating the reduction of carbon content as a function of reducing chrome content at different sub-atmospheric pressures and at a given, relatively low, temperature.

As is apparent from FIG. 1 a comprehensive unit for refining steel in accordance with some of the features of the invention includes an inductance heated vessel 1, containing molten steel 2 of a known chemical composition in a pressure-controlled chamber 3 at subatmospheric pressures. A mechanical pressure reducing unit schematically indicated at 4 serves to provide the necessary sub-atmospheric pressure. The pressure is indicated by pressure control meter, such as illus trated in the form of a mercury manometer shown at 5;

The melt 2 used in the refining process is to be made by the standard process of melting steel scrap, coke and high carbon ferrochrome, or chrome iron alloys, obtained from an electric furnace, induction furnace, blast furnace, cupola or any'other method.

Simultaneously, a gas injection system consisting of oxygen tank 6, helium tank 7, pressure regulators 8, 9, gas manifold 10, glass flow meter 11, and a water cooled gas lance 12, extends through airtight cover 13 into chamber 3. A thermocouple, preferably of the platinum-platinum Rhodium type and protected by a silica tube as schematically indicated at 14, is connected to a milli-volt meter 16 which indicates the temperature, and vessel pressure can be adequatelycontrolled.

THE PROCESS uct of this step is a chrome-iron alloy containing 8 to- 20 percent chromium with a carbon content of perhaps 1 to 6 percent. Such high carbon contents are exceptionally difficult to remove except at high cost in with minimal residual oxygen and requiring little if any I money, time and expensive ingredients. This process takes place in the refining stage and produces a desirable product at low cost, retaining the desirable elements and reducing the undesirable carbon content to lower levels than present practices permit economically.

As 'a first step of this process the molten metal is transferred practically slag free, into a vessel preheated to at least 1,300C, said vessel being, of the type indicated at 1, suitable for refining in an airtight chamber, as schematically indicated at 3.

A sample of the melt is immediately taken for chemical analysis, which will dictate the extent of the oxygen blow as is presently practiced in the modern Basic Oxygen Furnace steelmaking methods.

The vessel 1 is sealed in chamber 3 by attaching cover plate 13 preparatory to the oxygen blow.

Pressure in chamber 3 is then reduced to one-half atmosphere by operating unit 4 which may be a vacuum pump or otherwise well known construction, which also serves to evacuate the gases or vapors produced during the refining process.

As soon as the necessary analysis, sealing and connections are made, the lance 12 is positioned and the oxygen at sonic velocity is turned on, and the volume regulated to the desired flow by operating valves 8 and 9.

As the oxygen flow is continued the carbon level of the melt is reduced. The pressure in Chamber 3 is controlled to produce an increasingly lower pressure in accordance with the quantity of exhaustible gases produced.

As the carbon content decreases, the pressure in the airtight chamber 3 is further reduced continually to lower sub-atmospheric levels, eventually reaching minimal absolute pressures depending upon the endproduct and the temperature desired.

An end to the oxygen blow is determined from the condition of the atmosphere over the melt.

The temperature of the melt 2 is measured by lowering the protected thermocouple 14 into the melt. The temperature is controlled by regulating the oxygen flow or by the addition of coolants (scrap, etc.) or exothermic materials (ferrosilicon etc.).

Toward the middle of the oxygen blow an inert gas such as helium from tank 7 is added to the oxygen stream. The reactant gas is initially 100 percent oxygen, the oxygen concentration in the added gas is reduced gradually as the carbon removal progresses, to zero percent, while the inert gas starts at zero percent and increases to 100 percent, while under continually decreasing sub-atmospheric pressure to as low as mm of mercury absolute.

The inert gas is then also used to remove excess carbon monoxide, to purge the chamber of the oxygen atmosphere, and to de-oxidize the melt.

After the oxygen is completely shut off, inert gas at sonic velocity is introduced to agitate the melt and thus to bring about a closer approach to chemical equilib- 'rium relative to pressure, temperature and carbon and chromium contents.

Air is then allowed .to enter the chamber. When the chamber reaches atmospheric pressure, the chamber 3 is opened and a sample of the melt 2'is removed for a final chemical analysis and the temperature of the melt measured.

Under experienced supervision, the melt reaches the desired composition and temperature, and is ready for casting in any standard method.

Based upon experimental data, the blowing vessel requires a volume of approximately 35 cu ft per net ton of melt under a decreasing volume flow as the refinement continues and the temperature of the melt during the final refining period should be controlled between a minimum of approximately 1,600C and a maximum of approximately 1,800C.

The evacuating or pressure reducing unit 4 needs to be capable of removingall gases produced and to further evacuate the chamber to an absolute pressure equal to at least as low as one-fiftieth of an atmosphere and preferably to 5mm of mercury as aforenoted.

The gas manifold 6, 7, 8, 9, 10, 11 is to be so designed to provide the oxygen needed to oxidize the carbon in the melt and so regulated as to provide an oxygen or inert gas flow or any combination thereof as desired.

The water cooled lance 12 is specifically designed to be kept at temperatures to prevent deterioration and to feed reactant and/or inert gas in any combination at or near sonic velocity.

The pressure measuring instrument 5 serves to monitor the actual operating pressure in the chamber 3 and makes it possible to regulate the pressure as dictated from experience.

The temperature measuring apparatus 16 is designed so that it may be immersed as needed, conventional controls are provided to regulate the temperature within the specified limits.

Additional provisions may be made for the stirring of the molten metal either by the oxygen and/or an inert gas blow or by induced means, all this without departing from the scope of this disclosure.

The process also requires rapid analyzing of the melt immediately following the after blow, to permit further refinement if necessary, to meet the required alloy specifications.

SAMPLE EXAMPLES OF ONE TEST Heat No. 6326 A charge of high carbon ferrochrome, engine blocks, coke and fluxes was melted in an open arc tilting furnace.

The steel scrap and coke were added after the ferrochrome melted. An analysis of the melt was made and its composition just prior to tapping was:

' Temp. "C %Cr. %C %Si %S l6l0C 17.1% 4.12% 1.90% 0.03%

The slag-free melt 2 was tapped into a vessel 1 pre-. heated to 1,300C. The cover was then positioned and sealed to the melt laden chamber 3. The temperature of the melt was taken and the oxygen lance 12 positioned over the center of the melt 2. The pressure reducing unit 4 was turned on, and when the vessel pressure reached approximately one-half atmosphere, the oxygen blow was begun. Immediately, a violent reaction between the oxygen and the melt began. The oxygen feed rate was controlled to keep the reaction within limits. As the carbon began to burn out, the pressure in the vessel fluctuated due to the formation of carbon monoxide. The oxygen flow was regulated to keep the pressure down. As the carbon level in the melt decreased, the exhausting unit continued to reduce the pressure down to lower level. The carbon oxygen reaction began to subside. The helium gas was then admitted into the oxygen stream at an increasing rate, and the oxygen flow rate reduced. A continuous observation was kept on the rate of pressure decrease. A sudden decrease or levelling off in pressure was watched for, as this indicated an approach to the end of the decarburization phase. With an almost total helium flow lasting several minutes, the excess and residual oxygen was evacuated. Air was then permitted to enter the chamber and the pressure permitted to reach atmospheric. A sample of the refined melt was then removed 1. In a process of refining a chrome alloy steel melt containing between approximately 8 and 20 percent chromium whereby excessive carbon content is reduced to minimal content with little or no loss of other desirable alloying ingredients, the steps of transferring an initial chrome alloy steel melt obtained from a fur-' nace, and containing approximately 1 to 6 percent carbon, into a crucible at or near 1,300C, sealing said crucible in a vessel connected to means for reducing the pressure in said vessel, injecting gaseous oxygen in the form of a high velocity stream directed toward the surface of the melt, while maintaining a pressure of less than one atmosphere over the melt at all times and continuously reducing said pressure, said gas being injected at a rate controlled to maintain the melt at a temperature between 1,600 and 1,800C, until the desired carbon concentration is approached, then further reducing the oxygen flow rate while slowly mixing said oxygen with an inert gas to an increasingly rich inert gas concentration, the oxygen flow being terminated when oxidation of carbon in the melt has reduced the carbon content to a desired level, maintaining the melt temperature between approximately 1,600C and 1,800C, and reducing the percentage of residual carbon to approximately between 0.002 and 0.08 percent depending upon the percentage of chrome being between approximately 8 and 20 percent, said melt temperature and flow of inert gas being maintained until oxygen and oxidation products of carbon are essentially completely removed from said vessel.

2. Process according to claim 1 wherein the absolute pressure over the melt is reduced to one-half atmosphere prior to the start of the oxygen injection, and is progressively furtherl reduced to an absolute pressure as low as 5mm mercury in order to attain the desired carbon level, while maintaining the melt within the 1,600C and 1,800C temperature range.

3. Process according to claim 1, wherein the carbon content is reduced to progressively lower levels with successive lowering of the absolute pressure to a pressure substantially lower than that required for initial oxygen injection and while the oxygen and the inert gas are mixed from percent oxygen progressively to 100 percent inert gas.

4. Process according to claim 1, wherein at the desired carbon level the oxygen injection is reduced and an inert gas is mixed with the oxygen in increasing percentage until a 100 percent inert gas injection is effected, said injections being effected in the form of a high velocity stream directed in a direction substantially perpendicular to the melt surface. 

1. IN A PROCESS OF REFINING A CHROME ALLOY STEEL MELT CONTAINING BETWEEN APPROXIMATELY 8 AND 20 PERCENT CHROMIUM WHEREBY EXCESSIVE CARBON CONTENT IS REDUCED TO MINIMAL CONTENT WITH LITTLE OR NOR LOSS OF OTHER DESIRABLE ALLOYING INGREDIENTS, THE STEPS OF TRANSFERRING AN INITIAL CHROME ALLOY STEEL MELT OBTAINED FROM A FURNACE, AND CONTAINING APPROXIMATELY 1 TO 6 PERCENT CARBON, INTO A CRUCIBLE AT OR NEAR 1,300*C, SEALING SAID CRUCIBLE IN A VESSEL CONNECTED TO MEANS FOR REDUCING THE PRESSURE IN SAID VESSEL, INJECTING GASEOUS OXYGEN IN THE FORM OF A HIGH VELOCITY STREAM DIRECTED TOWARD THE SURFACE OF THE MELT WHILE MAINTAINING A PRESSURE OF LESS THAN ONE ATMOSPHERE OVER THE MELT AT ALL TIMES AND CONTINUOUSLY REDUCING SAID PRESSURE, SAID GAS BEING INJECTED AT A RATE CONTROLLED TO MAINTAIN THE MELT AT A TEMPERATURE BETWEEN 1,600* AND 1,800*C. UNTIL THE DESIRED CARBON CONCENTRATION IS APPROACHED, THEN FURTHER REDUCING THE OXYGEN FLOW RATE WHILE SLOWLY MIXING SAID OXYGEN WITH AN INERT GAS TO AN INCREASINGLY RICH INERT GAS CONCENTRATION, THE OXYGEN FLOW BEING TERMINATED WHHEN OXIDATION OF CARBON IN THE MELT HAS REDUCED THE CARBON CONTENT TO A DESIRED LEVEL, MAINTAINING THE MELT TEMPERATURE BETWEEN APPROXIMATELY 1,600*C AND 1,800*C AND REDUCING THE PERCENTAGE OF RESIDUAL CARBON TO APPROXIMATELY BETWEEN 0.0002 AND 0.08 PERCENT DEPENDING UPON THE PERCENTAGE OF CHROME BEING BETWEEN APPROXIMATELY 8 AND 20 PERCENT, SAID MELT TEMPERATURE AND FLOW INERT GAS BEING MAINTAINED UNTIL OXYGEN AND OXIDATION PRODUCTS OF CARBON ARE ESSENTIALLY COMPLETELY REMOVED FROM SAID VESSEL.
 2. Process according to claim 1 wherein the absolute pressure over the melt is reduced to one-half atmosphere prior to the start of the oxygen injection, and is progressively further reduced to an absolute pressure as low as 5mm mercury in order to attain the desired carbon level, while maintaining the melt within the 1,600*C and 1,800*C temperature range.
 3. Process according to claim 1, wherein the carbon content is reduced to progressively lower levels with successive lowering of the absolute pressure to a pressure substantially lower than that required for initial oxygen injection and while the oxygen and the inert gas are mixed from 100 percent oxygen progressively to 100 percent inert gas.
 4. Process according to claim 1, wherein at the desired carbon level the oxygen injection is reduced and an inert gas is mixed with the oxygen in increasing percentage until a 100 percent inert gas injection is effected, said injections being effected in the form of a high velocity stream directed in a direction substantially perpendicular to the melt surface. 